The invention relates to an improved triaxial acceleration sensor using force-sensitive resonators or strain gauges.
A number of force-sensitive resonators are described in the prior art. Single vibrating beam force sensors are described in U.S. Pat. Nos. 3,470,400, 3,479,536, 4,445,065, 4,656,383, 4,658,174, 4,658,175, 4,743,790, 4,980,598, 5,109,175, and 5,596,145. Double vibrating beam force sensors referred to as Double-Ended Tuning Forks (DETF) are described in U.S. Pat. Nos. 2,854,581, 3,148,289, 3,238,789, 4,215,570, 4,372,173, 4,415,827, 4,469,979, 4,531,073, 4,757,228, and 4,912,990. The change in frequency of oscillation of the resonant force sensors is a measure of the applied force.
There has been considerable effort in the prior art to apply resonator technology to acceleration measurements. Since acceleration is a three-dimensional vector, there are potential technical and cost benefits to develop an inherent triaxial sensor instead of a system using three single-axis devices. Previous efforts to make an inherent triaxial acceleration sensor encountered difficulties with suspending an inertial mass from tension-compression members. If the mass is held from both sides, in a push-pull arrangement, as shown in Aerospace Sensor Systems and Applications by Shinuel Merhav (Springer 1996, p. 165), the device becomes temperature-sensitive since thermal expansion or contraction acts on the opposing members. Furthermore, it is difficult to hang a mass with tension-compression members from three sides in a stable structure without using suspension systems or flexures. These suspensions constrain the forces generated by acceleration to one or two axes, prevent the simultaneous measurement of the full acceleration vector, and degrade performance.
Single-axis accelerometers employing resonator beams are disclosed in U.S. Pat. Nos. 2,984,111, 3,190,129, 3,238,789, 3,440,888, 3,465,597, 4,091,679, 4,479,385, 4,980,598, 5,109,175, 5,170,665, 5,334,901, and 5,596,145. In general, these devices are open-loop sensors without servo feedback, consisting of an inertial mass that exerts a force on the resonator under acceleration along the sensitive axis. The inertial mass is usually guided by a suspension system or flexures. Since a portion of the acceleration-induced load is shared with the suspension system or flexures, performance is degraded. In U.S. Pat. No. 4,479,385 by Koehler, the inertial mass is suspended by two resonators without additional flexures. This device is statically quasi-stable, tends to deflect in the direction of acceleration, and is prone to low-frequency oscillations in the sensitive axis. These and other shortcomings are greatly improved upon with the present invention.
In the prior art, multi-axis accelerometers have been proposed to measure acceleration along any axis. Prior art triaxial accelerometers have not been successful in meeting the high performance of single-axis accelerometers. One type of triaxial accelerometers consists of coaxial tension-compression pairs of vibrating strings that hold a central inertial mass in three orthogonal directions, as disclosed in U.S. Pat. Nos. 3,002,391, 3,057,208, and 3,382,724. The strings must be pre-tensioned to sustain compressive loads and suffer from long-term drift as the strings relax and creep with time. These accelerometers are not statically determinate and are susceptible to moments and applied linear and torsional vibrations. The tension-compression, push-pull arrangement also makes the prior art devices temperature sensitive as thermal expansion generates forces on the strings.
Erdley discloses two types of three-axis accelerometers utilizing force-sensitive resonators in U.S. Pat. No. 3,238,789. One type consists of a central mass suspended in three directions by dual-beam resonators. This device can only function accurately in an environment that is completely free of external moments. It is not a statically determinate structure and is prone to torsional oscillations about each of its axes, leading to performance degradation and breakage. The other prior art device consists of three pairs of coaxial resonators, similar to the triaxial vibrating string devices. This device is sensitive to temperature because thermal expansion and contraction exert unwanted tensile and compressive forces on the resonators. Again, the device is not statically determinate and moments tend to excite low-frequency torsional vibrations leading to breakage or performance degradation.
The invention described here combines for the first time the advantages of open-loop sensor technology in a structural arrangement that is in static equilibrium, statically determinate, temperature insensitive, and inherently triaxial.
A sensor is disclosed for providing a measure of linear acceleration in three orthogonal directions. The sensor includes a single inertial mass that is suspended in three orthogonal directions by support members in a statically determinate structure. The support members are strain gauges or force-sensitive resonators whose frequencies of oscillation vary with load. These tension-compression members are arranged in a statically determinate way such that they counteract three linear forces and three moments generated by linear and angular acceleration applied to the inertial mass. Six support members may be used as six constraints, arranged in three pairs, one pair for each orthogonal direction, in triaxial symmetry. In one embodiment of this invention, the two support members in each direction are spaced apart and loaded equally, with the sum of the outputs being a measure of linear acceleration to the inertial mass. The set of signal outputs contains the complete information of all linear forces generated by acceleration acting on the inertial mass. It can easily be corrected for geometric alignment and is thus a highly accurate measure of acceleration in three-dimensional inertial space.